Let be the mean (in points) of four test scores, x1, x2, x3, and x4, all

Question:

Let be the mean (in points) of four test scores, x1, x2, x3, and x4, all in points.
a. Find a formula for the mean of the test scores.
b. If a student has test scores of 87, 92, and 86, find the score she needs on the fourth test so that her four-test mean score is 90 points, which is the cutoff for an A.